[Dovecot] per-mailbox message limits

This is something Timo hacked up for me a few years ago and I realised should be on the list in case anyone else wants them.

The following patches will limit maildir folders to 4000 messages on Dovecot 2.0.* and 2.1.*

The Specfile patch is against the Cityfan Redhat EL5 SRPM but is likely to work on most build platforms

Changing the message limit requires a recompile. It's brutal and ugly but it prevents users overloading FSes that can't cope with huge numbers of files (EG: GFS2)